Superconducting magnetic sensor technology is based on the fact that some materials lose all of their electrical resistance and exclude all external magnetic fields when they are cooled below a critical temperature. External magnetic fields are excluded by electrical currents which develop within the material, which in turn generates a magnetic field opposing and exactly canceling locally the external field. The absence of all electrical resistance results in the absence of thermally generated electrical noise. Because of this phenomena a superconducting loop of wire will carry an electrical current proportional to any externally applied field. The current in a closed loop can be measured (i.e., converted to a voltage) using a Superconducting Quantum Interference Device (SQUID) which is a device utilizing the Josephson effect discovered by Brian Josephson.
A magnetic field is a vector quantity, i.e., possessing both direction and magnitude. A single loop superconductor mounted on a plane will measure the vector field component normal to the plane. A vector magnetometer (i.e., a device that measures the 3 vector components of the local magnetic field) can be constructed by implementing three loops, one on each of three orthonormal planes.
In many applications the spatial derivatives of the magnetic field vector components are of interest. Each of the vector field components have 3 spatial derivatives forming a total of 9 components usually denoted as the elements of a second order tensor as ##EQU1##
Magnetic field properties dictate that the gradient tensor be symetric and traceless. Thus, the gradient tensor is completely specified by 3 off-diagonal components and 2 diagonal components. A diagonal component of the gradient tensor may be sensed by two superconductor loops which are separated and wherein the planes of such loops are coplanar. An off-diagonal component of the gradient tensor may be sensed by two superconductor loops which are separated in the same plane, which plane is orthornormal to each of those coplanar planes formed by those superconductor loops which do sense the diagonal component(s) of the gradient tensor.
One prior art structure for holding thirteen superconductor loops in a planar relationship so that the entire gradient tensor may be derived from the sensing of such loop physically resembles an "iron cross". The sensitivity of the gradiometer so implemented in an "iron cross" physical configuration is proportional to the area of the loops times the separation distance between the loops. Because this sensitivity coefficient has dimensions of length cubed it is termed a volume coefficient. The areas of each of the two loops for each gradiometer component must be identical. If they are not identical the output of the sensor will be proportional to a mixture of both gradient and field components. Because of these dependencies on separation distances and loop areas, the "iron cross" configuration gradiometer needs have physical dimensions which are poorly adaptable in size and form to most cryogenic dewars which are possessed of a cylindrical volume. When the "iron cross" configuration gradiometer is scaled to fit a given cylindrical dewar volume, then the gradiometer's sensitivity (i.e., the volume coefficients of each gradiometer) is smaller than that achievable using a cylindrical configuration for the gradiometer.
Such a cylindrical configuration gradiometer is also known in the prior art. Upon the surface of such a cylindrical configuration gradiometer it is known to create as few as five superconducting loop windings from the sensing of which such windings the entire gradient tensor may be resolved. The local magnetic gradient at the gradiometer can be represented by the independent gradient values g.sub.xx, g.sub.zz, g.sub.xy, g.sub.yz, and g.sub.xz --but these values are not obtained directly from the five gradiometer windings but are rather only derived therefrom. The sensor outputs for the five superconducting loop gradiometer windings are represented as B.sub.xx, B.sub.zz, B.sub.xy, B.sub.zy, and B.sub.xz. The relationship between the gradient values and the sensor outputs are a function of the physical loop winding geometries. In particular, in the prior art it is known to create two of these five loop windings, specifically the loop windings which sense B.sub.zz and B.sub.yz, in a pattern so that these loop windings are, respectively, sensitive to measure not only g.sub.zz and g.sub.yz gradients but these loop windings do also, each one, measure significant additive quantities of g.sub.xx in their outputs. For simplicity of processing the signals derived from the loop windings in derivation of all forms of the gradient tensor, and in order that error contributions from the measurement of g.sub.xx should not affect the sensor outputs B.sub.zz and B.sub.yz, it is desirable that the sensor loop pattern of a cylindrical tensor gradiometer should be improved.